A typical such apparatus uses a roller in which is seated at least one rocker-type measuring bar or beam generally flush with the cylindrical outer surface of the roller and supported on sensors, for example force measurers, preferably piezo elements. Such a roller can be used for determining if there are defects in the planarity of a strip, or for determining the planarity of a metal strip when the planarity-measuring bar integrated into the roller jacket or the roller surface is supported on one or more and extends obliquely with respect to the roller axis so that the measuring bar on the one hand extends axially over a specified transverse width region and on the other hand extends angularly over a specified angular extent of the roller. In such a planarity-measuring roller, a plurality of planarity-measuring bars may be distributed transversely of the longitudinal strip-travel direction, across the transverse width of the strip. Each such bar covers a respective transverse region of the measurement roller or the metal strip and optionally extends over an identical angular extent. Each planarity-measuring bar is preferably supported on two sensors set at the ends of the respective the bar for example. The sensors are preferably designed as force transducers, for example strain gaugs. However, the invention also encompasses embodiments using other sensors.
By use of such a planarity-measuring roller, defects in the planarity of strips, in particular metal strips, are determined by measuring the distribution of strip tension across the transverse width of the strip, the strip under tension over the entire strip width wrapping around the planarity-measuring roller at a specified wrap angle, thereby exerting localized pressure forces on the planarity-measuring roller corresponding to the local longitudinal tension distribution in the transverse strip width direction, from which the distribution of strip tension may be determined. The distribution of strip tension across the width of the strip thus indicates the locations of strip defects and in particular waves or strip sabers, since longitudinal deviations in individual flat strips are reflected in differences in tension.
A planarity-measuring roller of the above-described type is known from commonly owned U.S. Pat. No. 7,357,022. In this planarity-measuring roller, two measuring bars are integrated diametrically opposite each other into the roller and are connected together by a diametral tension rod. Each of the measuring bars is supported on force transducers. By using such a system, in particular piezoquartz elements under pretension may be operated as sensors without having to take force shunts into account, since pretension, centrifugal forces, and gravity forces from the measuring bar may be compensated for by the diametrically opposed configuration.
Although the known planarity-measuring rollers are well known, they are still amenable to refinement. Thus, there is the basic problem that the measuring bar is used to measure the tension at a given strip width coordinate which results from superimposition of the strip internal stress and the strip tension. This is because the strips are always led over the planarity-measuring roller at a specified strip tension. Lateral fluctuations in the strip tension may then lead to skewing of the measurement results. In principle it is possible to record the strip tension as a function of time, using a separate device, in order to generate a corrective factor. However, this entails a high level of complexity. If the strip tension is measured using, for example, conventional load cells in the region of the roller bearings, in many cases the dynamics are not adequate.
In addition, a roller is known having two strip-shaped pressure sensors on the outer surface of the roller, the pressure sensors being mounted in a central region of the roller and extending essentially angularly around the roller. The pressure sensors include an inorganic material having piezoelectric properties. Such a roller is a component of a printer, in which two rollers border a pressure gap (see DE 103 29 430 A1). Such developments have had no effect on the development of measurement rollers for determining the strip tension, and in particular of measurement rollers for determining defects in the planarity of a strip, in particular a metal strip.